This article appeared in a journal published by Elsevier. The attachedcopy is furnished to the author for internal non-commercial researchand education use, including for instruction at the authors institution

and sharing with colleagues.

Other uses, including reproduction and distribution, or selling orlicensing copies, or posting to personal, institutional or third party

websites are prohibited.

In most cases authors are permitted to post their version of thearticle (e.g. in Word or Tex form) to their personal website orinstitutional repository. Authors requiring further information

regarding Elsevier’s archiving and manuscript policies areencouraged to visit:

http://www.elsevier.com/copyright

Author's personal copy

Granulation of Fe–Al–Ce hydroxide nano-adsorbent by immobilization in porouspolyvinyl alcohol for fluoride removal in drinking water

Hai-Xia Wu a, Ting-Jie Wang a,⁎, Lin Chen a, Yong Jin a, Yu Zhang b, Xiao-Min Dou c

a Department of Chemical Engineering, Tsinghua University, Beijing 100084, Chinab Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, Chinac School of Environmental Science and Engineering, Beijing Forestry University, Beijing 100083, China

a b s t r a c ta r t i c l e i n f o

Article history:Received 24 July 2010Received in revised form 18 December 2010Accepted 11 February 2011Available online 18 February 2011

Keywords:GranulationAdsorbentPolyvinyl alcoholFluoride removalWater treatment

Nano-adsorbents of Fe–Al–Ce trimetal hydroxide (FAC) were immobilized in porous polyvinyl alcohol (PVA)via cross-linking with boric acid. Spherical composite granules of 3–5 mm in size that would not cause a largepressure drop in a packed bed were obtained. SEM images showed that the FAC particles were embedded inholes of about 10 μm in the PVA granules, while the surface pores of the granules were only nano-scale in size.Thermal analysis showed that the FAC and PVA in the granules combined tightly by forming a chemical bond.The mechanical stability of the granules decreased with increased FAC concentration, and increased withincreased PVA concentration. The fluoride adsorption capacity of the granules increased with FACconcentration and decreased with PVA concentration. For acceptable mechanical stability and adsorptioncapacity, a FAC concentration of 12% and PVA concentration of 7.5% were suggested. The adsorption capacityof the granules prepared under suggested concentrations was 4.46 mg/g at an initial fluoride concentration of19 mg/L and pH 6.5. Immobilization of the nano-adsorbent in porous polyvinyl alcohol granules is a promisinggranulation method for water treatment in packed beds.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

High concentration of fluoride above 1 mg/L in drinking water willeasily causes dental and skeletal fluorosis [1], and WHO give a guidelimitation of less than 1.5 mg/L [2]. Adsorption is considered one ofthe most efficient technologies for fluoride removal in drinking waterwhen compared with other technologies like reverse osmosis [3],nanofiltration, electrodialysis [4] and Donnan dialysis [5]. Many kindsof adsorbents have been investigated for efficient and economicaldefluoride removal. The most commonly used adsorbents areactivated alumina and activated carbon. However, they have lowadsorption capacity, poor physical integrity, require acidification andpretreatment and their effectiveness for fluoride removal reducesafter each regeneration [1]. As natural materials cannot meet indust-rial requirements, i.e., low cost and high adsorption capacity, man-made adsorbents have been developed. However, most adsorbentsare synthesized as fine powders or hydroxide floc [6]. Ultra-finepowder adsorbents cannot be used directly for water treatmentbecause of their low hydraulic conductivity (high pressure drop) [7] inpacked bed as well as in leaching, and because these adsorbentswould have to be used in devices that make the separation andrecycling of the adsorbent difficult and costly. In a packed bed, an

increase in the size of the granules can increase the hydraulic con-ductivity [8]. Therefore, powder granulation to produce granules withhigh mechanical stability is necessary for practical application.

Conventional granulation of powder adsorbents includes moldingand calcination. However, calcination at high temperature results in alow adsorption capacity of the granules because many active sites onthe adsorbent surface may be passivated. A Fe–Al–Ce trimetal hydro-xide adsorbent (FAC) was reported to have high adsorption capacity.After this was granulated to 1 mm pellets, the adsorbent still showedpromising performance in column experimental tests [6].

A granulationmethod that used an iron oxide adsorbent coating ona particle carrier to remove toxic ions in drinking water has beenreported. This could be operated in packed beds and it reduced thecost of the adsorbent [9,10]. The fluoride adsorption capacity formanganese–oxide-coated alumina (MOCA) reached 2.85 mg/g [11].However, the coated layer shed easily leaving the granules with littleadsorption capacity, and caused secondary pollution to the drinkingwater because of its low mechanical stability. At the same time, theratios of coated-layer to carrier were below 1% in the reportedliterature [12–14], which greatly limited the adsorption capacity ofthe coated adsorbent particles. Copolymer latex has been introducedas a binder to reinforce the bonding between adsorbent and carrier.The coating stability and adhesion strength of the adsorbent layer onthe carrier were improved [15].

An ideal adsorbent granulation process should produce granuleswith the optimal size and porous structure to give a packed bed with a

Powder Technology 209 (2011) 92–97

⁎ Corresponding author. Tel.: +86 10 62788993; fax: +86 10 62772051.E-mail address: wangtj@tsinghua.edu.cn (T.-J. Wang).

0032-5910/$ – see front matter © 2011 Elsevier B.V. All rights reserved.doi:10.1016/j.powtec.2011.02.013

Contents lists available at ScienceDirect

Powder Technology

j ourna l homepage: www.e lsev ie r.com/ locate /powtec

Author's personal copy

higher hydraulic conductivity and large surface area. The polymerencapsulation technique can provide porous structures that allowsubstrates to diffuse rapidly into the internal pores, which are oftenused for cell immobilization [16–19]. Polyvinyl alcohol (PVA) is acheap and nontoxic synthetic polymer, and its granules can meet themechanical stability requirements of waste water treatment [20].

This work studied granulation by immobilizing FAC in porouspolyvinyl alcohol granules to increase hydraulic conductivity inpacked bed. The feasibility of using PVA as amatrix for the granulationof FAC, and the FAC and PVA concentration effects on mechanicalstability and fluoride adsorption capacity were investigated.

2. Experimental

2.1. Materials

FeSO4·7H2O, Al2(SO4)3·18H2O and Ce(SO4)2·4H2O used wereanalytical grade (Chemical Engineering Company of Beijing, China).The polymerization degree of the PVA used was 1700 and theapproximate molecular weight of the PVA was about 74800, and thesaponification ratio was 99% (Beijing Yili Fine Chemicals Co., Ltd,China). The other chemicals used were analytical reagent (AR) grade.

2.2. FAC preparation

FeSO4·7H2O, Al2(SO4)3·18H2O and Ce(SO4)2·4H2Owere dissolvedin deionized water to form a mixed solution with concentrations of0.1 M, 0.2 M and 0.1 M, respectively. 6 M NaOH solution was slowlytitrated into the mixed solution until the pHwas 9.5. The solution wasstirred at 200 rpm during the whole process [6]. The precipitatesobtainedwere centrifuged andwashedwith deionized water until thepH of the filtrate was 6.5±0.2. The product FAC of trimetal hydroxidenano-adsorbent was kept in deionized water.

2.3. Granulation process

The granulation was carried out referring to the reportedimmobilization procedure [21,22]. First, the PVA was dissolved indeionized water at a set concentration and at 95 °C with stirring.Second, the FAC suspension and PVA solution were mixed to form asuspension for granulation. Third, boric acid was added to 20 g/Lcalcium chloride solution to form a saturated boric acid solution thatwas used as the crosslink reagent. Fourth, the mixed suspension ofFAC and PVAwas dropped into the boric acid solution for cross-linkingand immobilization. The gel granules were kept in the boric acidsolution at 20 °C for several hours in order to achieve sufficient cross-linking between PVA and boric acid. Then, the granules were washed,dried at room temperature and kept in a capped bottle. The reactionbetween the boric acid and PVAmolecular is depicted in the followingequation. The concentration of boric acid and PVA both affect thecross-linking process.

CH2

C H

C

CH2

H

CH2

OH

H2C

CH

C

H2C

H

H2C

OH

OH

H3BO3

H2C

CH O

C O

H2C

H

H2C B O 3H2O++2

In addition, the sapoinfication ratio of PVA should be over 99% fora high stability of the PVA granules, and this high stability can be

maintained even when the granules are soaked in high pH solutionover two months.

2.4. Fluoride adsorption test [6]

The fluoride solution was prepared by dissolving NaF (analyticalgrade) in deionized water at a concentration of 0.001 mol/L (19 mg/L)according to the concentration order of the fluoride-contaminatedunderground water. The adsorbent granule dose was 2 g/L. Thevolume of the fluoride solution was 100 mL. The pH of the testsolution was adjusted by 0.05 M HNO3 or 0.05 M NaOH solution untilit reached to 6.5. The solution was shaken at 100 rpm and at 25 °Cduring the adsorption process. The fluoride ion concentration of thesolution was measured with a fluoride selective electrode connectedto an ion meter (PXS-450, Shanghai Kang-Yi Instruments Co., LTD,China) during the adsorption process for over 20 h, after which thegranules reached the adsorption equilibrium.

The adsorption capacity, qe (mg/g), was calculated using the fol-lowing equation:

qe =C0−Cf

m× V ð1Þ

where C0 (mg/L) is the initial fluoride concentration of 19 mg/L,Cf (mg/L) is the final fluoride concentration which is measured by theion meter, V is the volume of the fluoride containing solution of100 mL, and m is the adsorbent granules dose of 0.2 g.

KNO3 solution with a concentration of 0.2 M was added to thefluoride containing water as the background electrolyte beforeadsorption. The introduction of electrolyte solution can maintain thesample solution at a same activity coefficient, to eliminate theinfluence of the different solution ionic strength on the measurementaccuracy. The fluoride concentration was measured during theadsorption process at intervals.

However, considering the competition adsorption between NO3−

and fluoride ion on the adsorbent granules during fluoride adsorption,the measured adsorption capacity according to above method was alittle lower than the actual adsorption capacity. Therefore, themeasured adsorption capacity was calibrated from the followingexperiments on different kinds of adsorbent granules. KNO3 solutionwith a concentration of 0.2 M was added to the solution after theadsorption process was completed and the adsorbent granules werefiltered, to eliminate the competition effect during adsorption. Thenthe final fluoride concentrations were recorded by the ion meter. Themean of the raising fluoride ion adsorbed onto the granules in theseexperiments was about 7% of the initial amount of the fluoride ionin the water. Therefore, the values of measured adsorption capacitywere calibrated by increasing 7% to approach the actual adsorptioncapacity.

2.5. Characterization

The morphology of FAC was examined by a high-resolutiontransmission electron microscopy (HRTEM, JEM-2011, JEOL Co.,Japan). The morphology and structure of the granules were examinedby high resolution scanning electron microscopy (HRSEM, JSM 7401,JEOL Co., Japan). The inner structure of the granulewas inspected fromthe cross-section obtained by cutting the granule (PVA, FAC–PVA)with a sharp knife. The samples were coated with a thin gold layerbefore HRSEM observation. Thermal behavior of the PVA, FAC andFAC–PVA granules were analyzed by differential scanning calorimetryand thermo-gravimetric analysis (TGA2050, TA Instruments, USA).The samples were kept under N2 protection during the whole heatingprocess to prevent the influence of oxidation. A heating rate of 10 °C/min was used from 35 to 600 °C.

93H.-X. Wu et al. / Powder Technology 209 (2011) 92–97

Author's personal copy

The ratio of the mass of FAC shed to the FAC–PVA granules masswas used to characterize the mechanical stability. Granules (m1)were added to 100 mL deionized water, and after 24 h of shaking at250 rpm in a mechanical shaker, the granules were filtered and driedat 50 °C for 24 h to obtain the remaining weight (m2). The stability ofthe granules was calculated from m2/m1×100%.

3. Results and discussion

3.1. Granulation process

The FAC particles were synthesized according to the reportedliterature. In the reported literature, the adsorption capacity was178 mg/g under an equilibrium fluoride concentration of 84.5 mg/L,with an adsorbent dose of 150 mg/L and at pH 7.0, and the desorptionefficiency of fluoride from the used FAC reached 97% with NaOHsolution at pH 12.2 [6]. Fig. 1 shows that the FAC size was on the nano-scale.

The mixture suspension of FAC and PVA was dripped using aperistaltic pump, and the droplets fell into a boric acid solution to becross-linked with boric acid. The droplets sank to the bottom of theboric acid solution due to their higher density, and were immobilizedin the sinking process.

Droplets contracted to spheres due to the interfacial tensionbetween the drop surface and the immiscible solution [23]. Experi-ments showed that the drop shape was affected by the drop's physicalproperties due to the differences in the mixture compositions.Spherical granules of 3–5 mm in size were obtained when the FACconcentration was between 4 and 12% and the PVA concentration wasbetween 7.5 and 10%, as shown in Fig. 2.

When the FAC concentration was between 4 and 12%, and the PVAconcentration was between 4 and 7.5%, a tail was formed on thespherical granule. When the FAC concentration was below 4% and thePVA concentration was below 4%, the suspension drops were difficultto granulate, and they tended to form rings. When the FAC concen-tration was above 12%, and the PVA concentration was above 10%, thesuspension could not form drops due to the high viscosity.

Therefore, the granulation research here used concentrations of4–12% for FAC and 4–10% for PVA. For mass production in the future,the preparation of the granules can be scaled up by numbering up ofthe nozzles [23].

3.2. Structure of the granules

The cross-linking process was analyzed by sampling the granulesat different times. The samples were washed and put in deionizedwater. The SEM images of the granule surface are shown in Fig. 3.Fig. 3a shows the image of the 10 min cross-linked granules, and

showed that the FAC had agglomerated. This occurred because theuncross-linked PVA is easily dissolved in water during washing andleaves the FAC agglomerated as the water evaporated. After 30 min,some cross-linked PVA spots could be seen in Fig. 3b. These spotsbecame more apparent after 2 h of cross-linking as shown in Fig. 3c.The surface became smooth and formed a film after 6 h of cross-linking as shown in Fig. 3d. Then the film was further cross-linked andpores formed after 12 h as shown in Fig. 3e. After 24 h, the PVA in thegranules has been sufficiently cross-linked and it formed a poroussurface. For high mechanical stability of the granules, a sufficientamount of time is necessary for cross-linking. In the following experi-ments, the set time for cross-linking was 24 h.

The surface and the cross section structures of a pure PVA granuleand FAC–PVA granule after 24 h cross-linking are shown in Fig. 4.Fig. 4a and b shows the outer surface and inner cross section structureof a pure PVA granule, respectively. It can be seen that tiny uniformlydistributed pores of nano-scale size were present on the outer surface,while the inner holes in the PVA granules shown as the cross sectionin Fig. 4b were about 10 μm, much larger than those on the outersurface. Fig. 4c and d shows the outer surface and inner cross sectionstructure of the FAC–PVA granules, respectively. It can be seen that theouter surface of FAC–PVA granule has the same sized pores as the purePVA granules, and the inner holes in the granule as shown in the crosssection in Fig. 4d were full of FAC particles.

The larger size of the granules gives the packed bed a highhydraulic conductivity, and the porous structure supports a largesurface area for adsorption.

3.3. Interactions of FAC and PVA

The interactions between FAC and PVA in the granules wereanalyzed by their thermal behavior during heating. Fig. 5 shows thecurves of weight loss in the PVA, FAC and FAC–PVA granules. In theTGA curve, the cross-linked pure PVA showed mass loss of a fewpercent below 150 °C, which was assigned to the evaporation ofadsorbed water. The PVA lost most of its weight in the range of 240–500 °C due to decomposition. The FAC nano-adsorbent lost about 27%of its mass below 600 °C, which was assigned to the loss of crystalwater.

Fig. 6 shows the DSC curves of the PVA, FAC and FAC–PVA granules.Considering the sharp weight loss of pure PVA granules in Fig. 5, anendothermic peak of 240 °C was assigned to PVA decomposition. Theendothermic peak of the FAC adsorbent at 110 °C was assigned to theevaporation of adsorbed water. The crystal water loss gave the broadendothermic process below 600 °C.

Fig. 6 shows that the endothermic peaks of the FAC–PVA granuleswere higher than 295 °C, which was much higher than the decom-position temperature of 240 °C for pure PVA granules. Although, theFig. 1. TEM image of the FAC nano-adsorbent.

Fig. 2. Spherical granules produced.

94 H.-X. Wu et al. / Powder Technology 209 (2011) 92–97

Author's personal copy

peaks of FAC–PVA were somewhat different, which may have beencaused by the different reaction quantity between PVA and FAC, thedifference were very small compared to the peak difference betweenthe FAC–PVA and pure PVA granules (about 55 °C). It can be inferredthat FAC and PVA had formed a chemical bond in the FAC–PVAgranules, instead of being just physically embedded. This made theFAC particles adhere tightly to the PVA polymer and the particlescould not be easily leak out.

3.4. Stability of the granules

The effects of FAC and PVA mixture concentrations on themechanical stability of the granules are shown in Fig. 7. Fig. 7 showsthat the mechanical stability of the granules decreased with increasedFAC concentration given a constant PVA concentration, which wasalso confirmed in the subsequent experiments. However, with

increased PVA concentration, the mechanical stability increased,which is also shown in Fig. 7. However, the stability of almost allthe granules was over 85%.

Experiments showed that with decreased PVA concentration, largepores and holes formed, which caused the FAC particles to easily leakout. When the FAC concentration was increased and PVA concentra-tion remained the same, the adhering strength between FAC andPVA decreased. Therefore, the increase of the FAC concentration anddecrease of the PVA concentration leads to a decrease in the mecha-nical stability of the granules.

3.5. Adsorption capacity of the granules

The adsorption capacity of the granuleswith different FAC and PVAcomponents was measured by detecting the amount of residualfluoride in the solution with a fluoride ion meter. The effect of FAC

Fig. 3. SEM image of FAC–PVA granules at different cross-linked time. a: 10 min; b: 30 min; c: 2 h; d: 6 h; e: 12 h; f: 24 h.

95H.-X. Wu et al. / Powder Technology 209 (2011) 92–97

Author's personal copy

concentration on the fluoride adsorption capacity of the granules isconfirmed and shown in Fig. 8. It shows that the adsorption capacityper unit mass of granules increased linearly with the FAC concentra-tion at a set PVA concentration, which indicated that the efficiency ofthe FAC nano-adsorbents was almost constant. It was also determinedthat a higher FAC concentration in the granules will give granules ahigher adsorption capacity.

Fig. 8 also shows that thefluoride adsorption capacity decreasedwithincreased PVA concentration given set FAC concentrations. A lower PVAconcentration in the suspension will give a higher adsorption capacityfor thegranules.However, a lower PVAconcentrationwill result in lowergranule stability, thus, a suitable PVA concentrationwould be one that isa balance between stability and adsorption capacity.

For an acceptable adsorption capacity and mechanical stability ofthe granules, a FAC concentration of 12% and PVA concentration of7.5% for the suspensionwas suggested. Under this condition, sphericalgranules of 3–5 mm can be obtained. The granules have high hydrau-lic conductivity in a packed bed, high adsorption capacity, and highstrength even when soaked in NaOH (pH 12) for over a month. Withthese granules, the fluoride adsorption capacity is 4.46 mg/g (gran-ules) at pH 6.5 and initial fluoride concentration of 19 mg/L.

The above results have demonstrated that the granulation of a FACadsorbent can be achieved by using porous PVA as an adsorbentmatrix. These granules can be used in packed beds for fluorideremoval from drinking water. However, for its application in industry,the durability, regeneration and the cost all need to be further studied.

Fig. 4. Structure of the granules. a, b: outer surface and inner cross section structure of the PVA granules; c, d: outer surface and inner cross section structure of the FAC-PVA granules.

100 200 300 400 500 6000

20

40

60

80

100

Wei

ght

loss

, %

Temperature, oC

PVA FAC FAC6% + PVA10% FAC9% + PVA10% FAC12%+ PVA10% FAC12%+ PVA7.5%FAC12%+ PVA 5%

Fig. 5. TGA curves of different granules.

100 200 300 400 500 600Temperature, oC

FAC

PVA

FAC6% + PVA10%

240 oC

295 oC

FAC9% + PVA10%

FAC12% + PVA10%

FAC12% + PVA7.5%

FAC12% + PVA 5%

End

o---

Exo

Fig. 6. DSC curves of different granules.

96 H.-X. Wu et al. / Powder Technology 209 (2011) 92–97

Author's personal copy

4. Conclusions

FAC was granulated by immobilizing FAC in a porous polyvinylalcohol matrix via cross-linking with boric acid, into spherical shapesof 3–5 mm in size. The 3–5 mm size gives the granules a high hy-draulic conductivity in packed beds for fluoride removal fromdrinking water. Experiments showed that the FAC were embeddedin the PVA in granule holes of about 10 μm, while the surface pores ofthe granules were only on the nano-scale, in a structure that supportshigh surface area for adsorption of fluoride ions. FAC and PVA hadformed a chemical bond, which made the FAC tightly loaded on theporous structure of PVA. The mechanical stability of the granulesincreased with increased PVA concentration, and decreased withincreased FAC concentration. A higher FAC concentration and lowerPVA concentration produced a higher adsorption capacity of thegranules. A FAC concentration of 12% and PVA concentration of 7.5%were suggested for the granulation and gave the granules an adsorp-tion capacity of fluoride ions of 4.46 mg/g at pH 6.5 and an initialfluoride concentration of 19 mg/L.

Acknowledgement

The authors wish to express their appreciation of the financialsupport of this study by the National High Technology Research andDevelopment Program of China (863 Program, No. 2007AA06Z319)and the National Natural Science Foundation of China (NSFC No.20906055).

References

[1] Meenakshi, R.C. Maheshwari, Fluoride in Drinking Water and Its Removal, J.Hazard. Mater. 137 (1) (2006) 456–463.

[2] WHO, Guidelines for Drinking-Water Quality, 1st Addendum to Third Edition,20068 http://www.who.int/water_sanitation_health/dwq/gdwq3rev/en/.

[3] M.S. Onyango, H. Matsuda, T. Alain, Chapter 1 fluoride removal from water usingadsorption technique, Adv. Fluorine Sci. (2006).

[4] M.A. Menkouchi Sahli, S. Annouar, M. Tahaikt, M. Mountadar, A. Soufiane, A.Elmidaoui, Fluoride removal for underground brackish water by adsorption on thenatural chitosan and by electrodialysis, Desalination 212 (1–3) (2007) 37–45.

[5] H. Garmes, F. Persin, J. Sandeaux, G. Pourcelly, M. Mountadar, Defluoridation ofgroundwater by a hybrid process combining adsorption and donnan dialysis,Desalination 145 (1–3) (2002) 287–291.

[6] X.M. Wu, Y. Zhang, X.M. Dou, M. Yang, Fluoride removal performance of a novelFe–Al–Ce trimetal oxide adsorbent, Chemosphere 69 (11) (2007) 1758–1764.

[7] T.L. Theis, R. Iyer, S.K. Ellis, Evaluating a new granular iron oxide for removing leadfrom drinking water, J. AWWA 84 (7) (1992) 101–105.

[8] M.J. Keyser, M. Conradie, M. Coertzen, J.C. Van Dyk, Effect of coal particle sizedistribution on packed bed pressure drop and gas flow distribution, Fuel 85 (10–11) (2006) 1439–1445.

[9] C.H. Lai, C.Y. Chen, B.L. Wei, S.H. Yeh, Cadmium adsorption on goethite-coatedsand in the presence of humic acid, Water Res. 36 (20) (2002) 4943–4950.

[10] S.L. Lo, T.Y. Chen, Adsorption of Se(IV) and Se(VI) on an iron-coated sand fromwater, Chemosphere 35 (5) (1997) 919–930.

[11] S.M. Maliyekkal, A.K. Sharma, L. Philip, Manganese-oxide-coated alumina: apromising sorbent for defluoridation of water, Water Res. 40 (19) (2006)3497–3506.

[12] J. Lukasik, Y.F. Cheng, F. Lu, M. Tamplin, S.R. Farrah, Removal of microorganismsfrom water by columns containing sand coated with ferric and aluminumhydroxides, Water Res. 33 (3) (1999) 769–777.

[13] C.H. Bolster, A.L. Mills, G.M. Hornberger, J.S. Herman, Effect of surface coatings,grain size, and ionic strength on the maximum attainable coverage of bacteria onsand surfaces, J. Contam. Hydrol. 50 (3–4) (2001) 287–305.

[14] M.S. Al-Sewailem, E.M. Khaled, A.S. Mashhady, Retention of copper by desertsands coated with ferric hydroxides, Geoderma 89 (3–4) (1999) 249–258.

[15] H.X. Wu, T.J. Wang, X.M. Dou, B. Zhao, L. Chen, Y. Jin, Spray coating of adsorbentwith polymer latex on sand particles for fluoride removal in drinking water, Ind.Eng. Chem. Res. 47 (14) (2008) 4697–4702.

[16] Y.J. Wang, X.J. Yang, H.Y. Li, W. Tu, Immobilization of acidithiobacillus ferro-oxidans with complex of PVA and sodium alginate, Polym. Degrad. Stab. 91 (10)(2006) 2408–2414.

[17] L.S. Zhang, W.Z. Wu, J.L. Wang, Immobilization of activated sludge using improvedpolyvinyl alcohol (PVA) gel, J. Environ. Sci. 19 (11) (2007) 1293–1297.

[18] Z.E. Long, Y.H. Huang, Z.L. Cai, Immobilization of acidithiobacillus ferrooxidans bya PVA-boric acid method for ferrous sulphate oxidation, Process Biochem. 39 (12)(2004) 2129–2133.

[19] B. Krajewska, Application of chitin-and chitosan-based materials for enzymeimmobilizations: a review, Enzyme. Microbiol. Technol. 35 (2–3) (2004)126–139.

[20] D. Bezbradica, B. Obradovic, I. Leskosek-Cukalovic, B. Bugarski, V. Nedovic, Immo-bilization of yeast cells in PVA particles for beer fermentation, Process Biochem. 42(9) (2007) 1348–1351.

[21] A. Idris, N.A.M. Zain, M.S. Suhaimi, Immobilization of Baker's yeast invertase inPVA–alginate matrix using innovative immobilization technique, Process Bio-chem. 43 (4) (2008) 331–338.

[22] I.S. Chang, C.I. Kim, B.U. Nam, The influence of poly-vinyl-alcohol (PVA) charac-teristics on the physical stability of encapsulated immobilization media foradvanced wastewater treatment, Process Biochem. 40 (9) (2005) 3050–3054.

[23] X. Li, T.J. Wang, Y. Jin, Granulation process for producing spherical particles of arubber antioxidant in a water cooling tower, Chem. Eng. Technol. 29 (10) (2006)1273–1280.

2 4 6 8 10 1280

85

90

95

100St

abili

ty, %

Concentration of FAC, %

PVA, 5%

PVA, 7.5%

PVA, 10%

Fig. 7. Effect of FAC concentration on the stability of the granules.

2 4 6 8 10 12 141

2

3

4

5

6

7 PVA, 5%

PVA, 7.5%

PVA, 10%

FAC concentration, %

F- a

dsor

ptio

n ca

paci

ty, m

g/g

F_: 0.001mol/L

Granules: 2g/L

Fig. 8. Effect of FAC concentration on fluoride adsorption capacity.

97H.-X. Wu et al. / Powder Technology 209 (2011) 92–97